DE 2608933 (BASF) discloses a low pressure (e.g. 9 bar), metallocene catalysed polymerisation step for ethylene. The temperature may be varied to control molecular weight. Lower temperatures provide higher productivity and higher molecular weights. There is no suggestion of pressure as a major influence on the polymerisation process.
EP 69593 (Mitsui) performs a special form of solution polymerisation with phase-separation using non-metallocene Ziegler-Natta catalyst systems. Significant amounts of solvent are present. A solution is formed having an upper cloud point for the polymer/monomer mixture in the reaction medium, such as an inert hydrocarbon, which is liquid under the prevailing conditions. Polymerisation is carried out above a so-called upper cloud point.
EP 109530 (EC Erdolchemie) discloses a low to medium pressure (up to 400 bar) gas phase polymerisation conducted above the melting point of the polymer with ethylene feed gas and molten polymer flowing in opposite directions. Particulate catalyst systems are used including metallocene-type transition metal components and alumoxane co-catalyst components. There is no agitation by mechanical means or turbulence of the molten polymer and a settling step is not necessary to coalesce the discontinuous polymer-rich phase.
EP 260999 (Exxon) discloses a high pressure ethylene polymerisation process at temperatures over 120.degree. C. and at least 500 bar using a bis cyclopentadienyl-transition metal compound and alumoxane as a catalyst system. Zirconocenes are used in the Examples. The Al/Zr ratio varies widely in the Examples (See Table 4). Example 18 uses 200.degree. C. and 500 bar for homopolymerisation of ethylene. The experiments are performed in 100 ml steel autoclaves in a batch procedure. The productivity of the catalyst system cannot be estimated reliably due to variations in catalyst performance. The Al/Zr ratio of 29390 gives low productivity (productivity as used herein indicates the amount of polymer produced per total amount (Al+Zr) of catalyst metal) caused by the high Al content of the catalyst system. Other Examples illustrate continuous procedures at lower Al/Zr ratios but process conditions particularly pressure are such that the reactor contents are above the cloud point.
DE 3150270 (EC Erdolchemie) also claims broad temperature and pressure ranges using an Al/transition metal ratio in the Examples of circa 2500 but does not specifically teach any combination thereof which gives rise to two-phase conditions.
In a paper given by G. Luft in 1989 at the Hamburg Macro-molecular Symposium, high pressure polymerisation is performed at 1500 bar with Al/Zr ratio's well in excess of 10000 (ten thousand). As in DE 2608933, temperature is recognised as a major influence on the process. Lower temperatures are associated with higher catalyst productivity and higher molecular weight but the polymerisation rate is lower as a result of the lower temperature.
EP 416 815 uses low pressures but solvent present provides 1-phase conditions.
EP 399 348 uses low pressures but these either are 1-phase (solution) or involve temperatures below the melting point (suspension or gas-phase polymerisation). Also WO 88/05792 does not describe a two phase condition above the polymer melting point.
With gas-phase systems, the polymerisation rate is limited by the concentration of polymerised polymer pellet and the monomer concentration as well as the monomer diffusion through the polymer pellet growing around the supported catalyst particle.
The monomer feed in gas-phase operation has to be recirculated and conversion at each pass is limited by the permissible adiabatic temperature increase. Polymers with higher amounts of comonomer or low molecular weight product cannot be conveniently made if process conditions become unsatisfactory.
With high-pressure systems, it has generally been believed that high temperatures and pressures are necessary to obtain conditions conducive to good productivity of the catalyst system. To the extent that the art refers to lower pressures it is generally to delimit a lower end of a very broad pressure range. Actual examples in the art concern high pressures.
It is hence desirable to provide a process for polymerising olefinic feeds having a low catalyst consumption, energy requirement and capital costs but capable of producing a variety of polymers.